Such a circuit arrangement is disclosed in U.S. Pat. No. 3,678,199. The pulses causing the generation of a signal by the signal generator may be line synchronizing, field synchronizing and, also interference pulses, the reference level of said pulses being usually only slightly different from the black level of the video information. However, the threshold value has been chosen such that only the field synchronizing pulses are of a sufficiently long duration to produce a signal which exceeds the threshold value.
In this prior art circuit arrangement the threshold value must be chosen to be rather high to enable a somewhat reliable distinction between a field synchronizing pulse and a different, shorter pulse. If the incoming signal contains much noise then the threshold value will, however, not always been reached, while the signal produced by the signal generator between two consecutive pulses is not zero. This may cause the field synchronization to get lost. Furthermore, even in the event of little noise a hum voltage may be superposed on the television signal, which hum voltage may originate from, for example, an intermediate frequency amplifier, causing a direct current shift of the incoming signal, so that there is a risk that the threshold value is not reached at the proper moment.